Semicircular jewelled headdress (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
tiara
(Other definitions for tiara that I've seen before include "High diadem" , "Woman's crescent-shaped ornamental headdress" , "Jewelled head ornament" , "Jewelled headpiece" , "It bejewels" .)